if all you want is an amp to power the sub, then 1 preout is all you need. with 1 pair of RCA you can power 2ch...with 2 pairs you can power 4 chs...and so on.

if a HU has 3 sets of RCAs...one will be called the sub-preout. this is usually give you the option of adjusting the volume from the amp from the HU. the other 2 set will be for the 4ch amp which will let you control the balance/fade of the speakers.

although i have never hooked up an amp to a factory deck using the speaker level inputs or converters....im sure its not a difficult task. someone else should give you better insight on it.

i say sell the changer...get a deck with MP3 compatibilities.

that item you linked up there will allow you to have 4chs...giving you the option of hooking up a sub amp, or a 2ch amp, or even both, all with the RCA-less factory deck.